I. Field of the Invention
This invention relates generally to semiconductor wafer holders used in the storage, transportation and processing of semiconductor wafers. More particularly, it relates to a hermetically sealable purgable cassette used for containing semiconductor wafers during processing, transportation and storage having recess-forming pairs for supporting semiconductor wafers. The cassette of the present invention helps reduce warpage during the plastic injection molding and cooling process that is used in making the cassette, and during use of the cassette.
II. Discussion of the Prior Art
Production of semiconductor wafers requires an extremely clean environment. The presence of any small particles, vapors or static discharge in the environment is damaging to the production of semiconductors, and the wafers themselves. In an effort to combat airborne particle problems, various techniques are in use today.
The most common technique in use today, is to provide a cassette for holding the semiconductor wafers which meet Semiconductor Equipment and Materials International (SEMI) standards. The SEMI standards recite specifications with the purpose of defining interchangeable, standardized containers suitable for standardized processing cassettes. A standardized mechanical interface (SMIF) system has been proposed by Hewlett-Packard Company as disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389 with the purpose of reducing particle fluxes on the semiconductor wafers.
In the SMIF system, a cassette meeting SEMI standards is placed inside a clean Work In Process (WIP) box or pod. The SMIF pod or box keeps the cassette and wafers free of particles during transportation and storage. The pod or box also isolates the wafers from the operator. The semiconductor wafer cassette, the wafers, and the inside of the box or pod must all be free of particles damaging to the semiconductor manufacturing process.
The SMIF pods or boxes are used during processing in either a general clean room environment or a clean mini-environment (i.e.: under a canopy). Once in a clean environment, the SMIF boxes or pods are opened for removal of the cassette and for processing of the wafers. This can be done in a clean environment without contaminating the cassette or wafers.
During the automated processing of the semiconductor wafers a robotic arm of the processing tool removes the wafers from an indexed cassette and returns the processed wafers to their proper position. To prevent damage to the semiconductor wafers, when being removed and returned to the cassette by the robotic arm, the cassette must uniformly position a plurality of wafers within the cassette according to parameters pre-programmed into the automated process tool. For example, each wafer's horizontal alignment and the wafer's tilt is pre-programmed into the automated processing tool. The robotic arm then advances to this exact location to remove or return the semiconductor wafers.
When placing wafers inside the cassette, a recess-forming pair or divider or slot-forming spacer means supports and separates the wafers a predetermined distance from each other. This distance is also known by the robotic arm. During the molding process used to make the cassette and during use of the cassette, these recess-forming pairs may warp thereby causing the wafers to tilt outside the parameters pre-programmed into the automated processing tool. To remove the wafer, the robotic arm aligns itself within the cassette according to the pre-programmed parameters. If the wafer is tilted or not in horizontal alignment, the robotic arm may crash into the wafer, thereby damaging or destroying the semiconductor wafer.
Likewise, when replacing the wafer, if the divider has warped and therefore is not in the pre-programmed position, the wafer may scrape or crash against the warped divider. This damage or destruction to the semiconductor wafers can prove costly. Therefore, there is a need for a standard acceptable cassette having dividers that do not effect wafer transfer when warpage occurs.
The warp and shrink of the recess-forming pairs may be caused by several factors including: the polymer compound used, the selected press parameters for the plastic injection mold, part design, the total volume of each recess-forming pair, the thickness of the cassette sides, and the volume of the perimeter lip. Currently, with fiber filled resins, control of warping and shrinking during the plastic injection molding process is very difficult, if not impossible. Even when running the mold press within acceptable parameters, a significant percentage of molded cassettes have warped dividers that do not fall within the tolerances necessary for the wafer processing. This results in an over abundance of warped, unacceptable cassettes in the cassette manufacturing process. The molding of unacceptable cassettes increases the total cost, and production time of the molding process. Therefore, a need exists for a cassette that is not as readily effected by the above listed warp factors during the plastic injection molding and cooling process.
To permit automated processing of the wafers, the cassette must be indexed with the process tool. An H-bar is formed on the outside of a standardized cassette, which aids in the indexing of the cassette with the process tool. However, over time the H-bar warps and does not consistently index a cassette in the same location relative to the process tool. Therefore, the H-bar is not a dependable method for attaining high repeatability in indexing the cassette to the process tool.
During automated processing, the indexing of the cassette with the process tool must be performed in a clean environment to prevent contamination. In present manufacturing systems, the SMIF pod or box must be opened and the cassette removed in such a way that requires additional process tools and steps. The increase in required steps and tools adds to the likelihood that the process tool equipment will be unable to interface with the necessary removal tools.
The increase in required steps also requires a greater cycle time to index a cassette with the process tool. Further, when using a SMIF pod or box, the loading height of the process tool equipment must be large enough to allow for the removal of the standard cassette from the box or pod. Consequently, the SMIF pods or boxes are large and heavy requiring more storage space and increasing the likelihood of carpal tunnel damage to the handlers of boxes or pods. These problems are all overcome by eliminating the need to use SMIF boxes or pods. Elimination of the boxes or pods also reduces the amount of necessary clean-up, thereby further reducing the cost of processing semiconductor wafers.
The present invention overcomes the disadvantages of the current manufacturing system by providing a hermetically sealed cassette that can be purged with an inert gas. This sealed cassette may be indexed directly onto the process tool, eliminating the following steps: opening the SMIF box or pod, lowering the pod or box door simultaneously with the cassette, and manipulating the cassette onto the process tool. The hermetically sealed cassette has one surface of a three-groove kinematic coupling to positively locate the cassette with the process tool. This coupling provides a dependable method to position the cassette relative to the process tool with a high rate of repeatability.
Further, the cassette is smaller and about 20% to 50% lighter than the SMIF box or pod. Hence, there is a reduced likelihood that handlers will suffer from carpal tunnel. The reduced size eliminates the need that loading height of the process tool equipment be large enough for the removal of the standard cassette from a SMIF box or pod. Also, the required storage, transportation, and positioning space are reduced. A tracking system provided on the outer surface of the hermetically sealed cassette allows the cassette to be tracked in an unfriendly external environment during the transportation, storage or processing of the semiconductor wafers.
The negative effects of warpage, including the increased likelihood of semiconductor wafer damage, can be eliminated by providing a unique contour to each divider or recess-forming pair. Also, the addition of exterior ribs integrally corresponding with each contoured recess-forming pair reduces warpage of the contoured recess-forming pair during the plastic injection molding and part cooling process. The contoured recess-forming pair has a volume dependent on the corresponding exterior ribs, the cassette sides thickness, the distance between each recess-forming pair, and the volume of the perimeter lip.